linux-kernel - Re: [PATCH 1/2 v2] fdmap(2)

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Message-ID: <CALCETrWWUuzF7HaQAS2rf69KcqBrB9epvQPPgKEoK4MNbdVmEw@mail.gmail.com>
Date:   Sun, 24 Sep 2017 14:31:23 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Djalal Harouni <tixxdz@...il.com>,
        Alexey Gladkov <gladkov.alexey@...il.com>
Subject: Re: [PATCH 1/2 v2] fdmap(2)

On Sun, Sep 24, 2017 at 1:06 PM, Alexey Dobriyan <adobriyan@...il.com> wrote:
> From: Aliaksandr Patseyenak <Aliaksandr_Patseyenak1@...m.com>
>
> Implement system call for bulk retrieveing of opened descriptors
> in binary form.
>
> Some daemons could use it to reliably close file descriptors
> before starting. Currently they close everything upto some number
> which formally is not reliable. Other natural users are lsof(1) and CRIU
> (although lsof does so much in /proc that the effect is thoroughly buried).
>
> /proc, the only way to learn anything about file descriptors may not be
> available. There is unavoidable overhead associated with instantiating
> 3 dentries and 3 inodes and converting integers to strings and back.
>
> Benchmark:
>
>         N=1<<22 times
>         4 opened descriptors (0, 1, 2, 3)
>         opendir+readdir+closedir /proc/self/fd vs fdmap
>
>         /proc 8.31 ą 0.37%
>         fdmap 0.32 ą 0.72%

This doesn't have the semantic problem that pidmap does, but I still
wonder why this can't be accomplished by adding a new file in /proc.